In a fiber laser device, laser light is amplified by an amplifying optical fiber doped with rare earth elements and the amplified laser light is output from an end of the amplifying optical fiber.
Patent Document 1 listed below discloses an example of fiber laser device. In this fiber laser device, an end cap configured by a glass rod having a diameter larger than a core of the amplifying optical fiber is fused to an output end of the amplifying optical fiber. The light amplified by the amplifying optical fiber is input to the end cap from the amplifying optical fiber, a diameter of the laser light spot is increased in the end cap, and the light is output from another end surface, which is located on the opposite side of the amplifying optical fiber, from the end cap. The laser beam, having increased its diameter is of decreasing power density, and, as a result, it could have less damage threshold for an output end surface.
Patent Document 1: U.S. Pat. No. 7,190,511